Adaptive Immunity 1of2 Fall 2026

Overview

Robert Kellar, Ph.D., explains the adaptive immune system as a specific, memory-forming third line of defense, organized into antibody-mediated B-cell responses and cellular T-cell responses. He traces T-cell education and self-tolerance, antigen presentation through MHC I and MHC II, and antibody functions and isotypes, including IgA, IgD, IgE, IgG, and IgM.

Key takeaways

Adaptive immunity relies on specificity and memory: B and T lymphocytes are educated to respond to particular antigens and can respond more quickly when those antigens appear again.
T-cell activation requires two signals: antigen recognition and co-stimulation, including CD28 binding to CD80 or CD86; this checkpoint helps prevent inappropriate immune activation.
Central tolerance in the thymus and peripheral safeguards such as anergy, apoptosis, and regulatory T cells reduce the risk that self-reactive lymphocytes will cause autoimmune damage.
MHC class I typically presents antigens to CD8 cytotoxic T cells, while MHC class II presents antigens to CD4 helper T cells; MHC compatibility is also important in organ transplantation.
Antibodies do more than bind targets: they can neutralize toxins or microbes, promote phagocytosis, recruit natural killer cells, and activate complement.
The five major antibody isotypes have distinct roles: IgA protects secretions and mucosal surfaces, IgD is associated with B cells, IgE contributes to allergy and parasite defense, IgG dominates blood and crosses the placenta, and IgM is prominent early in primary responses.

Chapters

0:00 Exam 1 Format, Timing, and Study Priorities

Exam 1 covers material taught so far and has 50 questions in a one-hour Canvas session.
The exam opens at 7 a.m. and closes at 11:59 p.m. next Tuesday; students should leave a full hour and complete it in one sitting.
Lecture slides and class material are the main study focus; exam scores are retained, while the lowest pre- and post-quiz scores are dropped.

5:15 From Innate Defenses to Adaptive Lymphocytes

Lymphocytes are white blood cells associated with the lymphatic circulation and the adaptive immune system.
The first defense line includes skin, mucous membranes, eyelashes, and nasal hairs; acute inflammation is a useful second-line response.
Chronic inflammation can persist, cause fibrosis and tissue dysfunction, and involve foreign-body giant cells.

10:05 Adaptive Immunity, Specificity, Memory, and Vaccination

Adaptive lymphocytes become immunologically educated to recognize particular antigens and can retain memory for later exposure.
Vaccination uses an antigen exposure that cannot cause the full disease to induce protective antibodies or memory T cells.
The term vaccination traces to Latin vacca, meaning cow, and the historical observation that cowpox exposure protected against smallpox.

13:35 Humoral B-Cell and Cellular T-Cell Responses

Humoral immunity refers to antibodies circulating in body fluids, especially blood, and is driven by B cells and antibody-secreting plasma cells.
Cellular immunity uses CD4-positive helper T cells and CD8-positive cytotoxic T cells; CD4 and CD8 identify cell-surface clusters of differentiation.
Helper T cells coordinate immune activity with cytokines, while cytotoxic T cells can kill infected cells displaying a recognized antigen.

21:35 Adaptive Specificity and Two-Signal Co-Stimulation

An antigen-presenting cell provides signal 1 by displaying an antigen to a T-cell receptor; signal 2 is required for T-cell activation.
CD28 on a helper T cell binds CD80 or CD86 on an antigen-presenting cell to provide T-cell co-stimulation.
B-cell activation also requires co-stimulation; the lecture describes complement binding through CD21 as a supporting signal for humoral responses.

28:21 Thymus Development and Central T-Cell Tolerance

T-cell precursors originate in bone marrow and mature in the thymus; B cells mature in bone marrow and do not pass through the thymus.
The thymus grows through childhood, reaches its largest size around puberty, and gradually shrinks with age.
Central tolerance removes self-reactive T-cell clones through apoptosis, helping prevent attacks on tissues such as kidney or blood-vessel cells.

34:43 Peripheral Tolerance and the Cost of Clonal Selection

Peripheral tolerance provides backup for self-reactive T cells that escape thymic screening.
The described safeguards include anergy when co-stimulation is absent, apoptosis, and suppression by regulatory T cells.
Kellar estimates that about 95% of developing T cells are eliminated, leaving a much smaller population that passes tolerance checks.

39:23 Antigen-Presenting Cells Activate Specific T Cells

Macrophages, B cells, and dendritic cells can act as antigen-presenting cells and display processed antigens to T cells.
Activated CD4 helper T cells release cytokines that can stimulate macrophages and recruit further inflammatory activity.
CD8 cytotoxic T cells use antigen recognition and co-stimulation, including an interleukin-2 signaling step, to expand into active and memory populations.

43:30 Lymph Nodes Filter Fluid and Coordinate Immune Responses

Lymph nodes in regions such as the neck, armpits, and groin filter lymphatic fluid before it returns to venous circulation.
Dendritic cells carry antigens into lymph nodes, where they can initiate CD4 helper or CD8 cytotoxic T-cell responses.
Activated cytotoxic T cells can identify infected cells, such as respiratory epithelial cells, and destroy them so healthy tissue can regenerate.

50:05 Dendritic Cells, Macrophages, B Cells, and MHC Presentation

Dendritic cells in the skin, also called Langerhans cells, can capture antigens that pass the skin barrier and present them to lymphocytes.
MHC class I generally presents antigens to CD8-positive cytotoxic T cells; MHC class II generally presents antigens to CD4-positive helper T cells.
Macrophages digest microbes and display antigen fragments, while the MHC molecules on cells act as identity markers relevant to immune recognition.

55:56 MHC Compatibility, Organ Transplants, and Tissue Rejection

MHC differences between donor and recipient can contribute to transplant rejection, so clinicians test compatibility when matching a kidney donor.
Potential donors may include siblings, other relatives, and people on a transplant registry; a close match does not eliminate the need for immunosuppression.
The lecture contrasts mature cells with umbilical-cord Wharton’s jelly stem cells, which may have limited MHC expression before full development.

58:12 Antibody Structure and B-Cell Effector Functions

Antibodies have antigen-binding Fab regions and an Fc region that interacts with other immune components.
Antibody binding can neutralize microbes or toxins, promote opsonization and phagocytosis, recruit natural killer cells for antibody-dependent cytotoxicity, and activate complement.
B cells produce antibodies and can form memory B cells, while antibodies recruit other cells and processes to carry out much of the direct response.

1:02:36 IgA, IgD, IgE, IgG, and IgM: Isotypes and Roles

IgA is common in secretions and mucosal surfaces and can be secreted as a dimer; IgD is found on B-cell surfaces and at low levels in blood.
IgE participates in allergic reactions and defense against parasites; an overactive memory response to allergens such as peanuts can trigger hypersensitivity.
IgG makes up about 80 to 85% of circulating immunoglobulin and can cross the placenta; IgM is a pentamer prominent in the primary immune response.
Antibody valency is the number of antigen-binding sites: the familiar Y-shaped monomer has two, while IgA dimers and IgM pentamers have more.

Summary, takeaways, and chapters were generated by AI from the video's transcript and may contain errors. The video belongs to its creator, Robert Kellar, Ph.D..